Single Molecule Force Spectroscopy of Asphaltene Aggregates

Because the values of Lmax and Fmax obtained from a single force curve may ...... Sheremata, J. M.; Gray, M. R.; Dettman, H. D.; McCaffrey, W. C. Ener...
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Single Molecule Force Spectroscopy of Asphaltene Aggregates Jun Long,* Zhenghe Xu, and Jacob H. Masliyah Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2G6 ReceiVed December 29, 2006. In Final Form: March 7, 2007 Asphaltene aggregation and deposition cause severe problems in nearly all phases of petroleum processing. To resolve those problems, understanding the aggregation mechanisms is a prerequisite and has attracted the interest of a great number of investigators. However, to date, the nature and extent of asphaltene aggregation remain widely debated. In the present study, we attempt to investigate asphaltene aggregation from a completely new perspective. The technique of single molecule force spectroscopy (SMFS) was used to investigate the response of single asphaltene aggregates under an external pulling force. Force curves representing the stretching of single asphaltene aggregates were obtained in simple electrolyte solutions (KCl and calcium) and organic solvents (toluene and heptane). These force curves were well-fitted by the modified worm-like chain model, indicating that those asphaltene aggregates acted like long-chain polymers under pulling by an external force. It was found that lower solution pH values and the presence of divalent cations resulted in a lower bending rigidity of the formed aggregates. The information retrieved from the force curves suggests that asphaltene molecules with a structure featuring small aromatic clusters connected by aliphatic chains do exist and that asphaltene aggregation could occur through a linear polymerization mechanism. The current study extends the application scope of SMFS.

Introduction Asphaltenes are the most refractory fraction of crude oils. They are defined by their solubility in aliphatic solvents (i.e., they are the crude oil fraction soluble in toluene and insoluble in an n-alkane such as n-pentane or n-heptane). Hence, asphaltenes are a mixture of a great number of molecular species that consists of the most polar, most aromatic, and highest heteroatom components of the crude oils.1 One of the fundamental characteristics of asphaltenes is their tendency to self-associate in an organic solvent. The formation and deposition of asphaltene aggregates cause major problems in all phases of petroleum processing, including exploration, recovery, transportation, refining, and upgrading.2 With a continual increase in heavy and non-conventional oil production, these issues become acute. Understanding the mechanism of asphaltene self-association or aggregation and thus finding solutions to these problems are important and essential for the oil industry. Asphaltene aggregation in organic solvents has been observed and investigated for many years.3,4 A wide variety of techniques has been used in the investigation, including rheological measurements,5 surface tension measurements,6,7 small-angle X-ray scattering measurements,8,9 infrared spectrophotometry,10,11 * Corresponding author. E-mail: [email protected]. (1) Speight, J. G. The Chemistry and Technology of Petroleum, 3rd ed.; Marcel Dekker: New York, 1998; pp 412-467. (2) Rogel, E. Langmuir 2004, 20, 1003-1012. (3) Pferffer, J. P.; Saal, R. N. J. J. Phys. Chem. 1940, 44, 139-149. (4) Mullins, O. C.; Sheu, E. Y. Structures and Dynamics of Asphaltenes; Plenum Press: New York, 1998. (5) Sheu, E. Y.; Strom, D. A. Colloidal Properties of Asphaltenes in Organic Solvents. In AsphaltenessFundamentals and Applications; Sheu, E. Y., Mullins, O. C., Eds.; Plenum Press: New York, 1995; Ch. 1. (6) Rogel, E.; Leon, O.; Torres, G.; Espidel, J. Fuel 2000, 79, 1389-1394. (7) Sheu, E. Y. J. Phys.: Condens. Matter 1996, 8, 125-141. (8) Tanaka, R.; Sato, E.; Hunt, J. E.; Winans, R. E.; Sato, S.; Takanohashi, T. Energy Fuels 2004, 18, 1118-1125. (9) Tanaka, R.; Hunt, J. E.; Winans, R. E.; Thiyagarajan, P.; Sato, S.; Takanohashi, T. Energy Fuels 2003, 17, 127-134. (10) Gawrys, K. L.; Kilpatrick, P. K. Instrum. Sci. Technol. 2004, 32, 247253. (11) Aske, N.; Kallevik, H.; Johnsen, E. E.; Sjoblom, J. Energy Fuels 2002, 16, 1287-1295.

dielectric spectroscopy,12,13 differential scanning calorimetry,14 laser desorption mass spectroscopy,15 fractal structural studies,16 atomic force microscopic (AFM) imaging,17 and computer-aided simulation.12,18-20 Earlier studies5 indicated that asphaltenes can form micelle-like aggregates in aromatic solvents when the concentration of asphaltenes in the solvents is above the critical micelle concentration (CMC). Asphaltene aggregation is similar to the micellization of surface active materials (or surfactants) in aqueous solutions. However, some investigators21,22 did not observe any CMC during asphaltene aggregation and found that aggregation might originate from other mechanisms. Rather than the formation of micelles, Merino-Garcia et al.14,23,24 found that asphaltene aggregation in toluene can occur stepwise in a manner similar to polymerization. Agrawala and Yarranton25 also suggested an asphaltene aggregation model analogous to linear polymerization, which elucidated the results of asphaltene molar mass measurements and asphaltene phase behavior.25-27 In (12) Aguilera-Mercado, B.; Herdes, C.; Murgich, J.; Muller, E. A. Energy Fuels 2006, 20, 327-338. (13) Ortega-Rodriguez, A.; Duda, Y.; Guevara-Rodriguez, F.; Lira-Galeana, C. Energy Fuels 2004, 18, 674-681. (14) Merino-Garcia, D.; Andersen, S. I. J. Dispersion Sci. Technol. 2005, 26, 217-225. (15) Strausz, O. P.; Peng, P.; Murgich, J. Energy Fuels 2002, 16, 809-822. (16) Rahmani, N. H. G.; Dabros, T.; Masliyah, J. H. J. Colloid Interface Sci. 2005, 285, 599-608. (17) Toulhoat, H.; Prayer, C.; Rouquet, G. Colloids Surf., A 1994, 91, 267283. (18) Takanohashi, T.; Sato, S.; Tanaka, R. Pet. Sci. Technol. 2003, 21, 491505. (19) Takanohashi, T.; Sato, S.; Saito, I.; Tanaka, R. Energy Fuels 2003, 17, 135-139. (20) Rogel, E. Energy Fuels 2000, 14, 566-574. (21) Sztukowski, D. M.; Jafari, M.; Alboudwarej, H.; Yarranton, H. W. J. Colloid Interface Sci. 2003, 265, 179-186. (22) Yarranton, H. W.; Alboudwarej, H.; Jakher, R. Ind. Eng. Chem. Res. 2000, 39, 2916-2924. (23) Merino-Garcia, D.; Andersen, S. I. Pet. Sci. Technol. 2003, 21, 507-525. (24) Merino-Garcia, D.; Murgich, J.; Andersen, S. I. Pet. Sci. Technol. 2004, 22, 735-758. (25) Agrawala, M.; Yarranton, H. W. Ind. Eng. Chem. Res. 2001, 40, 46644672. (26) Yarranton, H. W. J. Dispersion Sci. Technol. 2005, 26, 5-8. (27) Alboudwarej, H.; Akbarzadeh, K.; Beck, J.; Svrcek, W. Y.; Yarranton, H. W. AIChE J. 2003, 49, 2948-2956.

10.1021/la063764m CCC: $37.00 © 2007 American Chemical Society Published on Web 04/19/2007

Force Spectroscopy of Asphaltene Aggregates

addition to the contradiction to the aggregation mechanisms, the reported structure, size, and shape of formed aggregates have been different from study to study.28 For example, the shape of asphaltene aggregates has been reported as spheres,29,30 disks,31,32 and cylinders.33 The intermolecular forces attributed to the aggregation were also reported to be diverse, including electrostatic interactions, van der Waals forces, intermolecular charge transfer, exchange-repulsion interaction, induction effects, hydrogen bonding, and the so-called π-π interactions.34,35 Considering the polydispersive nature of asphaltene molecules, the different sources of asphaltenes used, and the different approaches used in the study of asphaltene aggregation, such inconsistencies are not surprising. The previous discussion indicates that the nature and extent of asphaltene aggregation remain widely debated.26 In the present study, we attempt to study asphaltene aggregation using a completely new approach by employing the technique of single molecule force spectroscopy (SMFS).36 On the basis of the AFM technique, SMFS provides a possible way to precisely measure inter- and intramolecular forces on the scale of a single molecule.9 This technique has been successfully used to study the stretching of single biological macromolecules and synthetic polymer chains.37-40 In the current study, it was used to stretch single asphaltene aggregates pre-deposited on a substrate. In our study, the force to pull and extend a single aggregate as a function of the extension length was recorded. The information derived from the obtained force curves provided unique new insights into the aggregation mechanisms and the shape, size, and structure of the aggregates. The effect of liquid media on the structure of formed aggregates was studied by carrying out the force measurements in various aqueous solutions and organic solvents.

Langmuir, Vol. 23, No. 11, 2007 6183 Table 1. Characteristics of Asphaltenes Used in the Present Study Ma

C

H

N

O

S

E

H/C

fa

7072

79.71

8.25

1.20

1.33

7.81

10.34

1.23

0.50

a M: molecular weight (g/mol). C, H, N, O, S, and E: elemental composition (weight %) of carbon, hydrogen, nitrogen, oxygen, sulfur, and total heteroatoms (E ) O + S + N), respectively. H/C: hydrogen to carbon ratio; and fa: aromaticity of the asphaltenes.

Experimental Procedures Materials. Coker feed Athabasca bitumen was provided by Syncrude Canada Ltd. HPLC-grade toluene and n-heptane were purchased from Fisher Scientific. Ultrahigh purity KCl (>99.999%, Aldrich) was used as the supporting electrolyte, while reagent-grade CaCl2 (99.9965%, Fisher) was used as the source of calcium cations. Reagent-grade HCl and NaOH (Fisher) were used as pH modifiers of the aqueous solutions. Deionized water with a resistivity of 18.2 MΩ cm, prepared with an Elix 5 followed by a Millipore-UV Plus water purification system (Millipore Inc.), was used where applicable throughout this study. Asphaltenes were extracted from the Athabasca bitumen. The extraction and washing procedure and the characteristics of the asphaltenes obtained have been described in detail by Zhang et al.41,42 Table 1 gives the molecular weight, elemental composition, and aromaticity of the asphaltenes.41 Figure 1 shows a hypothetic molecular structure of Athabasca asphaltenes (adapted from Strausz (28) Sheu, E. Y. Self-association of AsphaltenessStructure and Molecular Packing. In Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998; pp 115-144. (29) Xu, Y. N.; Koga, Y.; Strausz, O. P. Fuel 1995, 74, 960-964. (30) Sheu, E. Y.; Liang, K. S.; Sinha, S. K.; Overfield, R. E. J. Colloid Interface Sci. 1992, 153, 399-410. (31) Herzog, P.; Tchoubar, D.; Espinat, D. Fuel 1988, 67, 245-250. (32) Ravey, J. C.; Ducouret, G.; Espinat, D. Fuel 1988, 67, 1560-1567. (33) Thiyagarajan, P.; Hunt, J. E.; Winans, R. E.; Anderson, K. B.; Miller, J. T. Energy Fuels 1995, 9, 829-833. (34) Murgich, J. Pet. Sci. Technol. 2002, 20, 983-997. (35) Murgich, J.; Rodriguez, J.; Aray, Y. Energy Fuels 1996, 10, 68-76. (36) Hugel, T.; Grosholz, M.; Clausen-Schaumann, H.; Pfau, A.; Gaub, H.; Seitz, M. Macromolecules 2001, 34, 1039-1047. (37) Ludwig, M.; Rief, M.; Schmidt, L.; Li, H.; Oesterhelt, F.; Gautel, M.; Gaub, H. E. Appl. Phys. A: Mater. Sci. Process. 1999, 68, 173-176. (38) Hugel, T.; Rief, M.; Seitz, M.; Gaub, H. E.; Netz, R. R. Phys. ReV. Lett. 2005, 94, 048301. (39) Liu, C. J.; Cui, S. X.; Wang, Z. Q.; Zhang, X. J. Phys. Chem. B 2005, 109, 14807-14812. (40) Janshoff, A.; Neitzert, M.; Oberdorfer, Y.; Fuchs, H. Angew. Chem., Int. Ed. 2000, 39, 3213-3237.

Figure 1. Hypothetical Athabasca asphaltene molecular structure.25,43 et al.).43 The main constituents of the molecule are a number of polyaromatic ring clusters connected by aliphatic chains. There are also such functional groups as acids, ketones, thiophenes, pyridines, and porphyrins (not all shown in Figure 1) with heteroatoms (N, O, and S) in the molecule. Similar structures of Athabasca asphaltene molecules were also proposed by Sheremata et al.44 Asphaltene Deposition on Substrates. To pick up a single asphaltene aggregate/molecule in the force measurements, a thin layer of asphaltenes had to be deposited onto a substrate. In the current study, newly cleaved mica sheets were used as the substrate. The deposition procedure was as follows. Asphaltenes were dissolved in HPLC-grade toluene at a concentration of 2 mg/mL. A newly cleaved mica sheet (∼10 mm × 10 mm) was immersed in 2-3 mL of the asphaltene-in-toluene solution in a beaker. The toluene was allowed to evaporate in a dust-free environment. During the evaporation process, the asphaltene concentration gradually increased, resulting in the formation of asphaltene aggregates attached to the mica sheet. This process continued until the toluene completely evaporated. The mica sheet was then taken out and washed using (41) Zhang, L. Y.; Xu, Z. H.; Mashyah, J. H. Langmuir 2003, 19, 9730-9741. (42) Zhang, L. Y.; Lawrence, S.; Xu, Z. H.; Masliyah, J. H. J. Colloid Interface Sci. 2003, 264, 128-140. (43) Strausz, O. P.; Mojelsky, T. W.; Lown, E. M. Fuel 1992, 71, 1355-1363. (44) Sheremata, J. M.; Gray, M. R.; Dettman, H. D.; McCaffrey, W. C. Energy Fuels 2004, 18, 1377-1384.

6184 Langmuir, Vol. 23, No. 11, 2007 deionized water to remove unattached asphaltene molecules. After it was dried by pure nitrogen blowing, the prepared substrate was attached to a puck for AFM force measurements. Force Measurement. A Nanoscope E AFM with a vendorsupplied fluid cell (Digital Instruments) was used for the force measurements. Silicon nitride probes also from Digital Instruments were used as received. Cantilevers of a lever type 200 µm with a spring constant of ∼0.06 N/m were used to measure the forces. The actual spring constant of the cantilevers was calibrated using the thermal tune method with a PicoForce AFM (Digital Instruments). Force measurements were performed in an aqueous environment or in an organic solvent in the fluid cell. The liquids used in the present study included deionized water, simple electrolyte solutions, and HPLC-grade heptane and toluene. After a test liquid was injected into the liquid cell, the system (the substrate, tip, and liquid) was left undisturbed for 1 h, allowing the system to reach equilibrium prior to force measurements. The procedure to obtain force curves is similar to the standard procedure of colloid force measurements.45-49 Briefly, a prepared substrate mounted on the AFM scanner was brought into contact with an AFM tip. Because of the nonspecific interactions between asphaltenes and the tip, one or more aggregates would attach onto the tip. During the retraction process or the separation between tip and substrate, the aggregate(s) connecting both the tip and the substrate were extended and stretched. The deflection of the cantilever as a function of the AFM piezo displacement was recorded and then converted to force versus separation distance curves. All force measurements were conducted at room temperature (23.0 ( 1.5 °C). Because it was difficult to control the number of molecules/aggregates picked up by the AFM tip, force curves featuring the stretching of a single molecule/ aggregate40 were obtained only through a great number of trials on different locations of the substrate surface. One important point that needs to be clarified here is whether the single molecule picked by the AFM tip is a single asphaltene molecule or a single asphaltene aggregate that behaves like a long-chain polymer. Based on Yen,50 the sizes of the asphaltenes increase due to association and aggregation as follows (numbers in parentheses are in nanometers): unit sheet or single molecule (1.2-2.0), stacks (3.0), aggregates (5.0), assemblages (10-15), clusters (200-2000), and flocs and spherules (1 × 103 to 20 × 103). Because the molecular weight of asphaltenes changes significantly from source to source and as a mixture, some of the asphaltene molecules could be quite large, and it is possible that the size (or length) of these asphaltene molecules exceeds 1.2-2 nm. However, it is unlikely that most asphaltene molecules have a size/length greater than several tens of nanometers. In the current study, the extension lengths obtained from the experimental stretching force profiles are in the range of 40-1200 nm. On the basis of Yen’s classification mentioned previously, it is clear that what the AFM tip picked up and then stretched in the force measurements is assemblages or clusters of asphaltene molecules. For the convenience of discussion, we used only aggregates in this paper to represent the aggregates, clusters, and even flocs that were referred to by Yen.

Force Curve Analysis Force Curve Correction. Figure 2a shows a typical plot of force versus separation distance obtained in deionized water. In the force measurements, each obtained force plot represents a whole cycle of tip-substrate approach and retraction. The inset of Figure 2a shows schematically the tip deflection on different stages of I-VIII during the approach and retraction processes. (45) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239241. (46) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 18311836. (47) Long, J.; Li, H.; Xu, Z.; Masliyah, J. H. AIChE J. 2006, 52, 371-383. (48) Long, J.; Xu, Z.; Masliyah, J. H. Energy Fuels 2005, 19, 1440-1446. (49) Long, J.; Xu, Z. H.; Masliyah, J. H. Langmuir 2006, 22, 1652-1659. (50) Yen, T. F. Asphaltenes. In Structures and Dynamics of Asphaltenes; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998; Ch. 1, pp 1-20.

Long et al.

Figure 2. (a) Pair of typical approaching and retracting force curves. The insets show the processes of the AFM tip approaching and retracting from the substrate. (b) Force curve obtained from both curves in panel a using -(Fretraction - Fapproach). This curve represents the extension of an asphaltene aggregate under pulling of the AFM tip.

The tip-substrate interaction forces for these stages are labeled correspondingly. When the tip was far away from the surface (I), there was no interaction between tip and substrate. As the tip approached the substrate, a repulsive long-range force was measured (II). This indicates that both tip and substrate could be negatively charged, leading to an electrostatic repulsion between them. The negative charge of the Si3N4 tip may arise from the ionized silanol (SiO-) and silylamine (SiN-) groups.51 The asphaltene substrate surface was also negatively charged due to the presence of such groups as RCOO-. As the tip was moved closer to the substrate, the repulsion rapidly increased (III). After the contact was made, the tip began to retract from the substrate (IV). The adhesion between tip and substrate resulted in a peak on the retraction force profile (V). After the tip pulled off the substrate, the force profile (VI) became reversed to that on approach (II) due to the electrostatic repulsion. At this point, a single asphaltene aggregate in a loose state might still be attached to both tip and substrate. As the retraction proceeded, an attractive peak was observed (VII), where the nonlinear increase of the force was due to the stretching of the aggregate in the z-direction. In region VIII, the tip was disconnected from the substrate and returned to its natural (undeflected) state. In the current study, the force we are interested in is solely the entropic and elastic force due to the stretching of individual (51) Ortiz, C.; Hadziioannou, G. Macromolecules 1999, 32, 780-787.

Force Spectroscopy of Asphaltene Aggregates

Langmuir, Vol. 23, No. 11, 2007 6185

Table 2. Results of Force Curve Fittings exptl results

fitting params

solution

Lmax (nm)

deionized water 1 mM KCl at pH 7.8 1 mM KCl at pH 3.2 1 mM KCl + 1 mM CaCl2 at pH 7.8 Heptane

100-330 90-100 40-100, 400-600 80-100, 1000-1200

0.15-0.3 0.2-0.4 0.5-1.5 0.75-2.5

∼0.75 ∼0.6 ∼0.25 ∼0.3

∼3.8 ∼4.2 ∼6.2 ∼26

300-900

0.3-0.6

∼0.32

∼3.5

Fmax (nN) lp (nm) K (nN)

asphaltene aggregates (here, entropic force is referred to as the force needed to decrease the entropy of the aggregates when the aggregates adopted a more ordered structure during the stretching process). However, the experimentally measured forces include all forces between tip and substrate, in particular, the electrostatic force. To obtain the net force due to chain stretching, the force due to the electrostatic interactions needs be eliminated from the retraction force profile by correcting it using a procedure suggested by Ortiz and Hadziioannou.51 Here, it is assumed that the electrostatic force on retraction is approximately equal to that on approach. By subtracting the forces measured on approach (Fapproach) from the measured retraction forces (Fretraction), the net force due to the chain stretching (F) was obtained, where

F ) -(Fretraction - Fapproach)

(1)

As attractive and adhesion forces are conventionally given as negative in AFM force measurements, in the previous equation, we use a negative sign before the subtraction so that the chain stretching force will be given as positive. Applying such manipulation to the force profiles shown in Figure 2a, a force curve representing the entropic and elastic force of a single asphaltene aggregate in deionized water as a function of its extension was obtained and plotted in Figure 2b. As the adhesion force between tip and substrate (V in Figure 2a) is not the focus of the current study, the peak representing this adhesion is not shown in Figure 2b. As shown in Figure 2b, two important values need to be noted: the final length of the extended aggregate (Lmax) and the maximum pulling force applied to the aggregate (Fmax, the rupture force). As the AFM tip could pick up an asphaltene aggregate at any position along the aggregate chain, the Lmax value obtained from the force curve represents only the length of part of the aggregate (not the length of the whole aggregate). However, it is still an important indicator of the magnitude of the aggregate length because the actual length of the aggregate must be at least equal to or in most cases larger than the Lmax value. The Fmax value, on the other hand, is also important. When the tip is detached from the substrate, three events can possibly occur: (i) desorption of the aggregate from the substrate, (ii) detachment of the aggregate from the tip, or (iii) breakage of the aggregate itself. Regardless of the event that occurs, the intermolecular interaction forces of asphaltene molecules in the aggregate must be at least equal to or larger than the Fmax value. Therefore, the Fmax value serves as an indicator of the strength of the intermolecular interaction forces that exist between asphaltene molecules in the aggregate. In the current study, the Lmax and Fmax values obtained under various test conditions are given in Table 2. Superimposition of Force Curves. As we did not have control over the exact location where the AFM tip picked up an asphaltene aggregate, the Lmax values varied from test to test under the same test conditions. To compare the results obtained, the force curves were therefore normalized by setting the extension as unity at a certain pulling force (e.g., 200 pN in this study). Figure 3

Figure 3. Superposition of typical force curves (inset) obtained in deionized water. The extension was normalized by setting it as unity at a pulling force of 200 pN for all force curves.

shows three normalized force curves obtained in deionized water. The corresponding original force curves are shown in the inset. A good superimposition of normalized force curves would indicate that the force curves obtained indeed represent the stretching of single molecules in general,40 and in our case, the stretching of single asphaltene aggregates. Theoretical Analysis of Force Curves. When an external pulling force is applied to a macromolecule chain, the molecule will be first aligned along the direction of the external force field and then be stretched if the force is sufficiently strong. The alignment is an entropy-controlled process during which the macromolecule changes its conformation. When the extension (x) of the macromolecule chain approaches its contour length (L), which is the length of the linearly extended molecule without stretching its backbone, further stretching the chain will exert a tension on the macromolecule backbone and result in deformation of bonds and bond angles. This is an enthalpy-controlled process. In general, the stretching behavior or elasticity of the polymer chain is affected by a number of factors, such as the interaction between the solvent and the chain52 and the salt concentration of the solution.36,53-56 For many polymers, the relation between the elastic elongation of the polymer chain and the external force can be quantitatively described by the worm-like chain (WLC) model.40 The WLC model describes a polymer chain as a string of constant bending elasticity with a worm-like conformation. By considering the enthalpic contribution, the following expression has been widely used to describe the force (F) as a function of chain extension (x)40

F(x) )

kBT 1 x F -2 x F 1 1- + + - lp 4 L K L K 4

[(

)

]

(2)

where L represents the contour length of the polymer chain, kB is the Boltzmann constant, T is the temperature, lp is the persistence length, indicating the stiffness/flexibility of the polymer chain, and K represents the specific stiffness of the polymer chain or (52) Oesterhelt, F.; Rief, M.; Gaub, H. E. New J. Phys. 1999, 1, 6.1-6.11. (53) Baumann, C. G.; Rouzina, I. F.; Bloomfield, V. A.; Smith, S. B.; Bustamante, C. Biophys. J. 1998, 74, A284. (54) Baumann, C. G.; Smith, S. B.; Bloomfield, V. A.; Bustamante, C. Proc. Natl. Acad. Sci. U.S.A. 1997, 94, 6185-6190. (55) Baumann, C. G.; Smith, S. B.; Bustamante, C.; Bloomfield, V. A. Biophys. J. 1997, 72, TH427. (56) Cui, S. X.; Liu, C. J.; Wang, Z. Q.; Zhang, X. Macromolecules 2004, 37, 946-953.

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Long et al.

Figure 4. Comparison of experimental results obtained in deionized water (solid line) with the fitting results (dashed line) by M-WLC model with lp ) 0.75 nm and K ) 3.5 nN.

the elasticity of a chain segment. This equation represents the modified worm-like chain model (M-WLC). In the present study, the previous statistic mechanics model for polymers was used to fit the force curves of single asphaltene aggregates. In the fitting practice, the adjustable parameters are lp and K. As for the contour length L, it is assumed to be close to the experimentally obtained length of the extended chain (Lmax). Ortiz and Hadziioannou51 found the following relations between the two lengths: Lmax ≈ 0.92L for the WLC model. In the current study, we found that the best fitted values of L were very close to the values of Lmax (i.e., Lmax ≈ 0.996L). Thus, the values of L were always set as L ) Lmax/0.996. Therefore, there are only two adjustable parameters in the fitting. The lp represents the flexibility (or bending rigidity) of the chain. It is mainly determined by the lower force portion (or the relative flat portion) of the force curves. On the other hand, K affects mainly the higher force portion of the force curves (i.e., where the force rapidly increases with extension until the rupture occurs). Hence, lp and K are relatively independent of each other, and thus for each force curve, unique values of lp and K are obtained. Figure 4 compares the experimental results obtained in deionized water (the same as shown in Figure 2b) with the fitting results by the M-WLC model. As shown by the dashed line in Figure 4, the force profile from the M-WLC model fits well with the experimental results (the solid line). In the current study, the M-WLC model was employed to fit all experimental results and the fitting results, including the values of lp and K for various test conditions, are given in Table 2.

Results and Discussion Effect of Solution pH. Solution pH is an important controlling parameter in many processes and operations of petroleum production. To find how solution pH affects asphaltene aggregation and the structure of formed aggregates, in the current study, we conducted force measurements at two different solution pHs (i.e., 3.2 and 7.8). Figure 5 shows two typical force profiles (solid lines) of stretching single asphaltene aggregates in 1 mM KCl solutions at pH 3.2 and 7.8, respectively. For comparison, a force profile obtained in deionized water (pH ∼6.5) was also plotted in this figure. From the three force profiles, the final length (Lmax) of the extended aggregates (or chains) was obtained to be about 93, 273, and 550 nm at pH 7.8, 6.5, and 3.2, respectively. The corresponding rupture forces (Fmax) are 301, 249, 534 pN. Because the values of Lmax and Fmax obtained from

Figure 5. Effect of solution pH on the stretching of single asphaltene aggregates in 1 mM KCl solution. (a) Experimental results (solid lines) and fitting curves (dashed lines) by the M-WLC model with lp ) 0.25 nm and K ) 6.2 nN for pH ) 3.2 and lp ) 0.60 nm and K ) 4.2 nN for pH ) 7.8. For comparison, the experimental and fitting results in deionized water (the same as Figure 3a) are also plotted. (b) Normalized force curves.

a single force curve may not be representative, a number of force curves for each test condition were obtained, and the ranges of Lmax and Fmax are given in Table 2. At pH 7.8, Lmax varies from 90 to 100 nm, and the rupture force (Fmax) is in the range of 0.15-0.3 nN. At pH 3.2, the Lmax value could reach between 400 and 600 nm, although in some cases, 40-100 nm were recorded. The Fmax value falls in the range of 0.5-1.5 nN. These results indicate that at a lower solution pH, longer asphaltene aggregates exist. Because asphaltene molecules are negatively charged at a solution of a pH larger than 4,57 there is an electrostatic repulsion between the asphaltene molecules. As a result of this repulsive intermolecular force, the larger aggregates deposited on the substrate might break into smaller ones during the force measurement when the substrate was immersed in the solution of pH 7.8. The presence of the repulsion also makes the net intermolecular interactions, which hold the asphaltene molecules together in the form of aggregates, appear to be weaker. At pH 3.2, the intermolecular electrostatic repulsion is weaker or may even disappear. Thus, larger aggregates still stayed on the substrate surface and could be picked up by the AFM tips. The decrease or absence of the electrostatic repulsion is also the reason why stronger rupture forces were detected. (57) Abraham, T.; Christendat, D.; Karan, K.; Xu, Z.; Masliyah, J. Ind. Eng. Chem. Res. 2002, 41, 2170-2177.

Force Spectroscopy of Asphaltene Aggregates

In Figure 5a, the dashed lines represent the results of theoretical fitting using the M-WLC model. The good agreement between fitted and experimental force profiles for all three cases indicates that the stretching process of single asphaltene aggregates in the aqueous solutions can be well-described by the M-WLC model, suggesting that the asphaltene aggregates behave like long-chain polymers in terms of their response to an external pulling force. The fitted persistence length (lp) is 0.25 and 0.6 nm for pH ) 3.2 and 7.8, respectively. These values show that the aggregates had a lower bending rigidity in an acidic solution than in an alkaline solution, suggesting that the chain was easier to coil in an acid solution and would tend to have a extended structure in an alkaline solution. This is also due to the presence of an electrostatic repulsion between asphaltene molecules. To compare the flexibility of the aggregates at different solution pH, the force curves were normalized and then plotted in Figure 5b. Clearly, in the range of the normalized extension from about 0.3 to 1, the force profile for pH ) 3.2 was always above that for pH ) 7.8. To stretch the aggregate to the same normalized extension, a stronger pulling force is needed in the solution of pH 3.2 than in the solution of pH 7.8. For example, at a normalized extension of 0.6, the force needed is about 10 nN for pH 7.8 and at least 20 nN for pH ) 3.2 as clearly shown in the inset. The difference between the two force curves obtained in the 1 mM KCl solution at pH 7.8 and in deionized water is negligible. Another point that needs to be noted is that the solution pH also affects the K value, which represents the specific stiffness of the aggregate chain or the segment elasticity. K can be understood as the inverse of the normalized compliance of a Hookean spring; the spring constant of the chain is given by K/L.36 At the lower pH of 3.2, the K value is about 6.2 nN, which is higher than the value of 4.2 nN obtained at pH 7.8. These results indicate that the segments of asphaltene aggregates became stiffer in an acidic solution than in an alkaline solution due to stronger intra- and intermolecular association in acidic medium. Effect of Calcium Cation. Calcium cations are normally present in connate water and industrial process water. Thus, understanding how calcium affects the aggregation behavior of asphaltenes is important. To that end, a calcium solution was prepared by adding 1 mM CaCl2 to a 1 mM KCl solution at pH 7.8. The stretching of single asphaltene aggregates was then conducted in the prepared calcium solution. Two typical force curves obtained in the presence and absence of calcium are shown in Figure 6a (solid lines). As shown by the dashed line, the fitting results by the M-WLC model agree reasonably well with the measured forces in the presence of 1 mM calcium. This finding indicates that under pulling by an external force, the asphaltene aggregate still responded like a worm-like polymer chain. The values of Lmax and Fmax directly extracted from the force curves and the fitted lp and K for the case of calcium addition are given in Table 2. In most cases, the Lmax value falls in the range of 1-1.2 µm (although sometimes a length of 80-100 nm was measured), indicating the existence of longer aggregates. The formation of longer (or larger) aggregates could be due to the bridging effect of calcium between the negatively charged asphaltene molecules. This effect also resulted in stronger rupture forces (Fmax). As given in Table 2, the value of Fmax in the presence of 1 mM calcium is in the range of 0.75-2.5 nN, much higher than the value of 0.2-0.4 obtained in the absence of calcium at the same pH of 7.8. As indicated by the smaller value of lp (0.3 nm) and the much higher value of K (26 nN), the presence of calcium made the asphaltene aggregates more likely in a coiled structure. This appears to be related to the reduction of the intermolecular electrostatic repulsion in the presence of

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Figure 6. Effect of calcium on the stretching of single asphaltene aggregates in 1 mM KCl solutions at pH 7.8. (a) Experimental results (solid lines) and fitting results (dashed lines) by the M-WLC model with lp ) 0.3 nm and K ) 26 nN in the presence of 1 mM calcium and lp ) 0.6 nm and K ) 4.2 nN in the absence of calcium. (b) Normalized force curves.

calcium. Figure 6b compares the normalized force curves. As clearly shown in the inset, the force needed to extend an asphaltene aggregate is higher in the 1 mM KCl solution with 1 mM calcium than in the solution without calcium. Results of Force Measurements in Organic Solvents. Asphaltenes are a solubility class, characterized as being insoluble in light n-alkanes (n-pentane or n-heptane) but soluble in toluene. The aggregation of asphaltenes in organic solvent has been the focus of many experimental and theoretical investigations.58 In the current study, we tried to investigate how the asphaltene aggregates pre-deposited on a mica substrate surface behave in tow organic solvents (i.e., n-heptane and toluene). Figure 7a shows a typical stretching force curve obtained in n-heptane. For comparison, a typical force curve obtained in deionized water was also plotted in this figure. Table 2 shows that the typical value of Lmax obtained in n-heptane varies from 300 to 900 nm. As compared to the Lmax value of 100-330 nm obtained in deionized water, the aggregates in n-heptane were longer. As asphaltene molecules in an aqueous medium such as deionized water could be negatively charged due to the ionization of various functional groups, the electrostatic repulsion between the asphaltene molecules would destabilize the long/large aggregates pre-deposited on the substrate, thus making them smaller. In contrast, asphaltene molecules in n-heptane are neutral in terms (58) Oh, K.; Ring, T. A.; Deo, M. D. J. Colloid Interface Sci. 2004, 271, 212-219.

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Figure 8. Proposed structure of asphaltene molecule with a condensed aromatic core (a) and a possible aggregate structure formed by such molecules (b).

of single asphaltene aggregates was obtained. This is because asphaltenes are soluble in toluene. When toluene was injected into the liquid cell where the force measurement was carried out, the aggregates pre-deposited on the mica substrate dissolved into the toluene phase. Thus, no aggregates were picked up by the AFM tip.

Figure 7. Stretching of single asphaltene aggregates in an organic solvent (heptane). (a) Experimental results (solid lines) and corresponding fitting results (dashed lines) by the M-WLC model with lp ) 0.32 nm and K ) 3.5 nN for the case of heptene and lp ) 0.75 nm and K ) 3.5 nN for the case of deionized water (the same as Figure 3a). (b) Normalized force curves.

of electric charge. Thus, the long aggregates were still present on the substrate surfaces and were picked up by the AFM tip. The same explanation can also be applied to the higher rupture forces obtained in n-heptane (0.3-0.6 nN, Table 2) than in deionized water (0.15-0.3 nN). In Figure 7a, the dashed lines represent the results of the M-WLC model. Comparing the fitted force curve with the experimental force profile, one notes that the stretching of the asphaltene aggregates in n-heptane can be well-described by the M-WLC model, indicating that the asphaltene aggregate under pulling still acted like a worm-like polymer chain in this organic solvent as in the aqueous media. Figure 7b shows the corresponding normalized force curves in n-heptane and deionized water. There is a clear difference between the two normalized force profiles. In the range of the normalized extension at about 0.4-1, the force profile obtained in heptane is always above that in deionized water, indicating that the aggregate had a higher tendency to coil in heptane than in deionized water. The M-WLC model fitting also shows such a trend. In the case of n-heptane, the fitted persistence length of lp is about 0.32 nm, which is smaller than the value of 0.75 nm for the case of deionized water. However, the K values in both cases are nearly identical, indicating that the segment elasticity of the asphaltene aggregate did not change much in these two liquid environments. We also conducted the single molecule force measurements in toluene. However, no force curve representing the stretching

Molecular Structure and Aggregation Mechanisms of Asphaltenes As discussed in the previous section, a typical force curve of stretching a single asphaltene aggregate not only provides information about the nanomechanical properties of the aggregate such as bending rigidity (lp) and segment elasticity (K) but also serves as indicators of the aggregate dimension and of the interand intramolecular interactions through the length of the extended chain (Lmax) and the rupture force (Fmax). All these properties/ results are directly related to the aggregate structure, which is controlled by the aggregation mechanisms. Therefore, one can in turn retrieve information about the mechanisms of asphaltene aggregation from the results obtained in the current study. Furthermore, the structure of the asphaltene molecules, which determines the aggregation mechanisms, can also be revealed to some extent. In this section, we discuss these matters collectively. Asphaltenes from different sources have been found to have similar measurable properties, such as elemental composition, functional group content, and density. However, their molecular structure could vary. Figure 1 shows a hypothetical structure of Athabasca asphaltene molecules. This structure features small aromatic clusters (two or more aromatic rings) connected by aliphatic chains. On the other hand, a structure consisting of a highly condensed aromatic core with small aliphatic chains on the periphery as shown by Figure 8a was proposed as the first model of asphaltene molecules.59 If asphaltene molecules have such large aromatic cores, they are likely to form colloidal stacks through the π-π bonds.59 The stacking of asphaltene molecules could result in the formation of cylindrical, spherical, and disklike aggregates.2 Figure 8b shows schematics of an aggregate formed through the molecular stacking mechanism. (59) Dickie, J. P.; Yen, T. F. Anal. Chem. 1967, 39, 1847-1852.

Force Spectroscopy of Asphaltene Aggregates

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Figure 9. Schematic mechanisms of asphaltene aggregation: (a) micellization and (b) linear polymerization.

For asphaltene molecules with a structure as shown in Figure 1 or with a similar structure proposed by Sheremata et al.44 (i.e., small aromatic clusters connected by aliphatic and sulfur side chains), they can link one another in various manners through aromatic stacking (π-π bonds), acid-base interactions, hydrogen bonding, and van der Waals interactions.25 In addition to the stacking mechanism (Figure 8b), two other typical aggregation mechanisms are linear polymerization and micellization.26 Figure 9 shows schematics of the two mechanisms. In Figure 9a, asphaltene molecules are treated like surfactants, and their micellization could result in the formation of small spherical aggregates of ∼3 nm.5 Large aggregates can then be formed by the micelles. Figure 9b shows the linear polymerization mechanism proposed by Agrawala and Yarranton25 and Merino-Garcia et al.14,23,24 In this scheme, the asphaltene molecules are considered to contain multiple active sites (heteroatoms or aromatic clusters) that interact with other molecules to form aggregates in a stepwise manner analogous to linear polymerization. Asphaltene aggregates formed through the three mechanisms would be different in their shape, size, and structure and thus in their nanomechanic properties (e.g., the bending rigidity or flexibility). Aggregates formed through the molecular stacking and micellization mechanisms would most likely assemble nanoparticles. It is difficult to stretch these particles using the SMFS technique. Even if they can be picked up and then stretched by an AFM tip, their bending rigidity would be very high (high persistence length, lp). This is because the particle-like aggregates would behave like a rigid object under the action of a pulling force of piconewtons. In the current study, the stretching force curves obtained for single asphaltene aggregates can be wellfitted by the M-WLC model, indicating that these aggregates acted like long-chain polymers. The values of the persistence

length, representing the bending rigidity, are quite small. The lowest value is about 0.25 nm (Table 2), which is close to the persistence length of polydimethylsiloxane in heptane (2.3 ( 0.02 Å)60 or that of polymethacrylic acid in aqueous solution (2.8 ( 0.5 Å).51 These two polymers have Si-O and C-C bonds as their backbones, respectively. The length of a simple C-C bond is about 1.54 Å, and the length of a Si-O bond is about 1.63 Å. Although there is no simple relation between the persistence length and the aggregate structure parameters such as bond length or the length of a monomer unit, the previous comparison indicates that stretching an asphaltene aggregate is analogous to stretching a linear polymer with C-C or Si-O bonds as its backbone. Two important conclusions can be drawn from such an analogy. First, the asphaltene aggregates that were picked up and stretched by the AFM tip had a structure similar to a long-chain polymer, suggesting that the asphaltene aggregates was formed through the linear polymerization mechanism. As we only presented the force curves exhibiting the feature of a single chain stretching, such a conclusion is not exclusive (i.e., aggregates could also be formed through other mechanisms). Second, the aliphatic links must exist between aromatic clusters in the asphaltene molecules because only with such a structure can the formed aggregates be stretched like a long-chain polymer of C-C backbone. This indicates that the asphaltene molecules in the aggregates should have a structure as shown in Figure 1 rather than that shown in Figure 8a. Another important point that needs to be addressed here is the possible implication of the rupture forces obtained. As given in Table 2, the value of the rupture force ranges from at least several (60) Senden, T. J.; di Meglio, J. M.; Auroy, P. Eur. Phys. J. B 1998, 3, 211216.

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hundreds of piconewtons to as high as 2.5 nN. As discussed earlier, the intermolecular interaction forces should be at least equal to or larger than the rupture force. This suggests that the aggregates were formed through binding of multiple active sites between asphaltene molecules, rather than a single bond, such as a hydrogen bond.

Conclusion In the present study, the technique of SMFS was used to investigate the stretching behavior of single asphaltene aggregates under the action of an external pulling force. Force measurements were carried out in simple electrolyte solutions (KCl and calcium) and organic solvents (toluene and heptane). Force curves representing the stretching of single asphaltene aggregates were obtained and well-fitted by the modified worm-like chain model, indicating that those asphaltene aggregates acted as long-chain

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polymers under the pulling of an external force. In a solution with a lower solution pH or with the addition of divalent cations, or in heptane, the formed asphaltene aggregates had a lower bending rigidity. The information retrieved from the force curves suggested that the asphaltene molecules with a structure featuring small aromatic clusters connected by aliphatic chains do exist and that the asphaltene aggregation could occur in a manner analogous to linear polymerization. The current study extends the application scope of SMFS. Acknowledgment. Financial support from NSERC Industrial Research Chair in Oil Sands Engineering (held by J.H.M.) is gratefully acknowledged. We also thank Dr. Liyan Zhang (now with Baker Petrolite) for kindly providing the asphaltene sample. LA063764M